Electron beam-pumped gas laser system

ABSTRACT

An electron beam-initiated-and-sustained gas laser system capable of producing uniform, high energy, large-volume electrical discharges in a high-pressure gaseous lasing medium. A thin metallic window separates the low pressure electron gun cavity from the longitudinally parallel high pressure gas cavity, such that the injected primary electrons create low energy secondary electrons which are swept across the gas cavity by an applied electric field collisionally exciting the gaseous medium. The system is applicable in both the oscillator and amplifier configurations, and is capable, for example, of producing a 10 joule/liter of active volume short (tp about 1 ns) CO2 laser pulse, while being operable in a manned area due to the housing being electrically neutral.

Krawetz et al.

[ ELECTRON BEAM-PUMPED GAS LASER SYSTEM y [75] Inventors: BartonKrawetz; Jack B. Long;

Leonard J. Mooney, all of Livermore, Calif.

[73] Assignee: The United States of America as represented by the UnitedStates Atomic Energy Commission, Washington, DC.

[22] Filed: Sept. 14, 1972 [2]] Appl. No.: 289,029

[52] US. Cl. 331/945 [5 l] Int. Cl. H01s 3/02 [58] Field of Search33l/94.5

[56] References Cited UNITED STATES PATENTS 3,702,973 11/1972 Daugherty331/945 1 Jan. 29, 1974 Primary Examiner-William L. Sikes Attorney,Agent, or Firm-John A. Horan; F. A. Robertson; L. E. Carnahan 5 7]ABSTRACT An electron beam-initiated-and-sustained gas laser systemcapable of producing uniform, high energy, large-volume electricaldischarges in a high-pressure gaseous lasing medium. A thin metallicwindow separates the low pressure electron gun cavity from thelongitudinally parallel high pressure gas cavity, such that the injectedprimary electrons create low energy secondary electrons which are sweptacross the gas cavity by an applied electric field collisionallyexciting the gaseous medium. The system is applicable in both theoscillator and amplifier configurations, and is capable, for example, ofproducing a 10 joule/liter of active volume short (tp ns) CO laserpulse, while being operable in a manned area due to the housing beingelectrically neutral.

5 Claims, 2 Drawing Figures GAS SUPPLY OR I D CONTROL ELECTRONBEAM-PUMPED GAS LASER SYSTEM BACKGROUND OF THE INVENTION The inventiondescribed herein was made in the course of, or under, Contract No.W-7405-Eng-48 with the United States Atomic Energy Commission.

The present invention relates to an electron beampumped gas lasersystem, and more particularly to such a laser system wherein largequantities of energy can be uniformly delivered to the lasing medium.

It is well established that large volumes of high pressure gaseouslasing medium can be pumped to high inversion densities by injecting anelectron beam of densities greater than a few milliamperes per squarecentimeter and at energies from an external source sufficiently large toproduce substantially uniform excitation, as exemplified by U. S. Pat.No. 3,641,454 issued Feb. 8, 1972, to Barton Krawetz, coinventor in thisapplication, and assigned to the same assignee.

SUMMARY OF THE INVENTION The invention constitutes an improved lasersystem over the system described and claimed in the abovereferenced U.S. Pat. No. 3,641,454 thereby resulting in improved power output.

Therefore, it is an object of the invention to provide an electronbeam-initiated-and-sustained gaslaser system.

A further object of the invention is to provide an electron beam-pumped,electrical-discharge, gas laser system wherein large quantities ofenergy can be uniformly delivered to a lasing medium.

Another object of the invention is to provide an improved electronbeam-pumped laser system wherein the electron generator is positionedlongitudinally parallel to the gaseous medium suchthat electronsuniformly traverse the gaseous medium along substantially the entirelength of the gaseous medium vessel.

Other objects of the invention will become readily apparent from thefollowing description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1, partially in cross-section,illustrates an embodiment of the invention; and

FIG. 2 schematically illustrates the system electronics.

DESCRIPTION OF THE INVENTION An embodiment of the inventiveelectron-beaminitiated-and-sustained electrical-discharge highenergy gaslaser system is illustrated in FIG. 1 in the amplifier configuration andbasically consists of a hot cathode electron-gun assembly and a gaseouslaser amplifier assembly indicated generally at and 11, respectively.The electron-gun assembly 10, capable of electron energies in excess of200 keV and current densities in excess of 200 mA/cm comprises a housingor casing 12 defining therein a chamber 13 having a longitudinal opening14 adjacent a longitudinal opening 15 in a container or vessel 16defining therein a cavity or chamber 17, casing 12 and vessel 16 beinginterconnected by a flange 18 of vessel 16. It is understood that casing12 and vessel 16 are electrically neutral and may be integral ifdesired, and may, for example, be constructed of stainless steel. Inthis embodiment the electron-gun assembly, as known in vacuum tubeterminology, is of the triode beam power tube type comprising a cathode19, an anode 20, and a control grid 12. Cathode 19, for example, may becomposed of 100, 20 cm long pure tungsten wires spaced 1 cm apart toachieve an active length of cm, and is connected to a power supply 22via lead 23 which extends through an insulative sealing or feedthroughmember 24 in casing 12, and is electrically negative with respect toground, with power supply 22 being at -l00 kV to -2 MV, for example, a200 kV being utilized in this embodiment. Anode 20 is secured to flange18 of vessel 16 which is electrically connected to ground, as indicatedat 25, and consists of a thin sheet or layer (1/2 to 10 mil) ofaluminum, titanium or a plastic such as KAPTON manufactured by E. I.DuPont, for example, forming in effect a window" between chambers 13 and17 transparent to electrons, and is supported by a hibachi grid orgrill-like member 28 which is secured to flange 18 of vessel 16 and thuselectrically grounded. In this embodiment a 0.0015 inch thick aluminumwindow is utilized and located 14 cm from the grid 21. Hibachi 28 isconstructed of material, such as stainless steel, and of a configurationso as to produce minimum interference with the electron flow betweencathode l9 and anode or window 20 while providing structural support foranode 20 in view of the thinness thereof and length which is about 1meter, for example. When the window 20 is constructed of plastic 1 V2mil KAPTON for example), a layer of aluminum several hundred (200 forexample) angstroms thick is plated on the KAPTON and connected to anelectrical potential (slightly positive with respect to ground in thisembodiment) to create a bias to repel positive ions formed in the gas inchamber 17, and to bleed off a static charge created on the window.Control grid 21 is located 2 cm from cathode 19, composed of 100, 40 mildiameter wires, and is electrically connected to a grid conrol generallyindicated at 30 via lead 31 which extends through an insulative orfeedthrough sealing means 32 in casing 12, grid control 30 operating inthe manner described hereinafter with respect to FIG. 2. If desired afocusing ring or shield about the outer edge either control grid 21 orcathode 19 and at the potential of the control grid or cathode,respectively, to focus electrons onto the window or anode 20 wherebyless dispersion of electrons results. Chamber 13 is under vacuum and ispumped down by a vacuum pump 33 connected to an opening 34 via conduit35 such that the base pressure, for example, is about 6 X 10"" torr withthe pressure at the time of firing the gun 10 being less than 1 X 10torr. The casing 12, constructed, for example, of stainless steel, isprovided on the interior surface thereof with a finish which allowsproper pump down of chamber 13. Several different pumping systems may beutilized and the vacuum pumping system may be integral with the casing12 and not remotely located as shown.

Vessel 16 of laser amplifier assembly 11 has, for example, a gas cavityof 10 cm radius (31.4 liter of active Cathode 39 functions to interceptarcs from anode 40 and thus provide protection for the thin window 20,and is positioned, for example, about 2 cm from window or anode 20 andis electrically connected to ground 25 via flange 18 of vessel 16,cathode or protective grid 39 being constructed of 14 gauge brass (0.083inch diameter) for example. Cathode or grid 39 may be connectedelectrically directly to window 20. Anode 40, constructed of stainlesssteel, for example, is connected to a power supply 42 via a lead 43which extends through an insulative seal or feedthrough 44 in vessel 16.For example, power supply 42 is at a positive potential of 50 k\/ to 300kV with respect to ground, 100 kV being utilized in this embodiment.Anode 40, for example, may be rounded at the edges thereof to reducearcing. Also, anode 40 may be provided with a blackened surface such aswith anodized aluminum or copper oxide to prevent selflasing effects.Chamber 17 is filled with a gaseous lasing medium such as CO N and He,or a mixture thereof at a pressure ranging from 1-l0 atmospheres. Inaddition, the rare gases Ne, Ar, Kr, Xe and Rn may be utilized providedhigh potential power supply for the electron gun 10 is utilized. Thechamber 17 is filled with the pressurized lasing media through an inletopening or port 45 connected via a conduit 46 to a gas supply 47 asindicated by the arrow, it being understood that appropriate valving isprovided to prevent gas being forced from the chamber back into thesupply system upon excitation of the laser. Pumpout of the gas fromchamber 17 is accomplished via an outlet port 48 connected toappropriate apparatus as indicated by the arrow at 49.

Referring now to FIG. 2, an embodiment of the electronics for the FIG. 1system are schematically illustrated. The negative 200 kV pulse 53 forelectron gun 10 from power supply 22 is derived from a six stage, 7.5 UMarx generator incorporating a series 50 ohm resistor for currentlimiting in case of an arc in the gun. Grid bias control (30) for thecontrol grid 21 is provided by a simple capacitor C1, resistor R1, sparkgap 861 combination. Capacitor C1 has a capacitance of l4.7 pf

while resistor R1 has resistance of IOOKQ. In practice the grid biascapacitor C1 is precharged to the desired voltage. Application of the200 kV pulse 53 then causes the spark gap 86] to overvoltage and biasthe grid. The 20 mil tungsten filaments of cathode 19 require 40 kW ofpower and are heated for 3 to 5 seconds before application of the 200 kVpulse. Cathode filament power is obtained from a variable powerstat Tlwhich drives a filament transform T2 that is isolated for 250 kV DCprimary to secondary. The powerstat Tl-filament transformer-T2combination is capable of 300 kW for 20 minutes on, 40 minutes off,thereby providing the ability to operate more than seven of the gunssimultaneously. A gas sustainer bank C2 for gas cavity anode 40 is up toa 50 pf, 300 kV bank without switching. Bank C2 is precharged to thedesired voltage (less than the voltage required to produce breakdown inthe gas in chamber 17) and is switched automatically by the e-beam 51entering the gas cavity or chamber 17.

slightly less than the breakdown voltage of the gas lasing medium inchamber 17. The chamber is pumped down to the base pressure and electrongun 10 is then turned-on to bring the cathode 19 up to operatingtemperature whereby chamber 13 remains at a vacuum, for example, ofabout 10* torr, whereup the grid control 30 is activated which activatescontrol grid 21 and the gun 10 fires causing a uniform flow or beam ofelectrons indicated by arrows 50, having an emission of l amp/cm, forexample, from the negative cathode 19 toward the window or anode 20which is at ground and thus positive with respect to cathode 19. Thebeam of electrons 50 is driven for ll0 psec duration, for example. Theelectrons 50 pass through thin window 20 and interact with the materialof the window and the lasing medium to produce secondary electrons 51which are attracted toward the positive anode 40 in such a manner as todisperse uniformly through substantially the entire volume of chamber17. Cathode 39 does not interfere with the secondary electrons and couldbe eliminated except that thin window 20 would be damaged by the largeelectrical discharge or arcs across chamber 17. The passage of the beamof electrons 51 through the gaseous lasing medium in chamber 17 causesionization of the lasing medium by electron collision thereby triggeringa large, uniform electrical discharge between electrodes 39 and 40. Boththe electron pulse (electrons 51) and the triggered electrical dischargeacross electrodes 39 and 40 excite the lasing medium, creating a largepopulation inversion, whereby a light beam passing through windows 37and 38 is amplified. For example, with the gaseous lasing mixture setforth above, the beam electrons and discharge electrons excite themixture to high-energy states by collision, whereby the excited Nresonantly transfers energy to the unexcited CO such that a populationinversion is generated in the lasing state of the CO, molecules andspontaneous emission from the excited CO, initiates lasing in theoscillator configuration, while in the amplifier configuration a lightbeam passing through windows 37 and 38 is amplified by stimulatedemission.

By the way of example only, and with the lasing medium being of the CO-N He mixture set forth above and at a pressure of about 3 atmospheres,the electric field between the electrodes 39 and 40 is about 5 to 15kv/cm, with the energy of the electron pulse from gun 10 being about200-5000 joules. Thus, a total energy content on the order of 20,000joules may be deposited in the gaseous mixture. Approximately 9% to 1percent of this energy is available for short pulse amplification givinga total energy output on the order of a few hundred joules in ananosecond light pulse, implying powers on the order of 10 megawatts inthe light.

With thepower supplies 22 and 42 atthe potentials indicated above andchamber 17 filled with a CO, mixture at a pressure of about 3atmospheres and with the emission of electrons 50 from cathode 19 of gun10 being about 1 amp/cm, the secondary electrons 51 have an energy ofabout 2-3 ev., the energy of electrons 51 being independent of thespecified emission of cathode 19.

While the invention has been illustrated and described in an amplifierconfiguration it can be utilized equally effective in an oscillatorconfiguration by providing an appropriate reflector member in properlyaligned and spaced relation with respect to each of Brewster anglewindows 37 and 38 so as to define an optical resonant cavity, as knownin the art. Also, while the electron gun assembly has been described andillustrated as being of a specific triode-type embodiment, the inventionis not limited to this gun configuration.

While the filaments of hot cathode 19 have been described as being ofpure tungsten, filaments of thoriated tungsten, tantalum and rhenium maybe utilized, which may provide more current density at lowertemperatures with better mechanical stability. Also, the interior ofvessel 16 and/or anode 40 can be blackened to prevent self lasingeffects by inserting sheets of materials such as anodized aluminum,honeycomb, felt, etc.

Safety factors provided by the inventive laser system enable the systemto be operated in a manned area, this being due primarily to the housing(casing 12 and vessel 16) being electrically neutral and of a wallthickness sufficient to provide x-ray shielding. In addition, view portsof lead glass of appropriate thickness may be provided in vessel 16without decreasing the safety factors.

While the inventive laser system has been described utilizing ahigh-pressure gaseous lasing medium, the system may also utilizelow-pressure gaseous lasing medium, thus are pressure range of 0.1 to 10atmospheres is possible with this system. Also, if desired asubstantially continuous laser pulse may be produced by the grid pulsecontrol activating the system at a faster pulse rate.

It has thus been shown that the present invention provides a structuralconfiguration which provides improved lasing capabilities over theelectron beampumped gas laser configuration covered by theabovereferenced U. S. Pat. No. 3,641,454.

What we claim is:

1. An electron beam-initiated-and-sustained electricaldischarge gaslaser system comprising: a vessel containing a gaseous lasing mediumunder pressure, said vessel comprising a housing defining alongitudinally extending closed chamber therein, Brewster angle windowstransparent to light at frequencies which the medium exhibits gainpositioned in opposite endwalls of said housing, a pair of electrodemeans positioned in spaced relation longitudinally within said chamber,said electrode means extending along a major portion of the longitudinallength of said housing, said pair of electrode means comprising alongitudinally extending cathode and a longitudinally extending anode,said cathode being connected electrically to ground and said anode beingconnected electrically positive with respect to ground, meanselectrically connected to said cathode and anode for impressingthereacross a voltage less than the breakdown voltage of saidpressurized gaseous lasing high energy medium, means for supplying saidchamber with said gaseous lasing medium, window means transparent tohigh energy electrons positioned longitudinally in said housing and inclose proximity to said cathode, said window means comprising a thinlongitudinally extending member defining a portion of a wall surface ofsaid chamber and extending along a major longitudinal portion of saidhousing, said window means being supported by a grill-like member, saidwindow means and said grill-like member being supported by said housingand electrically connected to ground, electron generating meanspositioned longitudinally parallel to and along a major portion of saidhousing adjacent said window means, said electron generating meansincluding a longitudinally extending casing secured to said housing soas to define a chamber in open communication along the longitudinallength thereof with said window means, means connected to said casingfor producing a vacuum in said last-mentioned chamber, said window meansdefining an anode of said electron generating means, cathode meanspositioned longitudinally in said casing in spaced relationship withsaid window means and connected electrically negative with respect toground, means for producing a voltage drop between said cathode meansand said window means including a negative potential power supplyelectrically connected to said cathode means, and means for controllingsaid voltage drop comprising a control grid positioned in said chamberintermediate said cathode menas and said window means and means forcontrolling said control grid, whereby activation of said electrongenerating means directs at least one beam of high-energy electronsalong substantially the longitudinal length and throughout substantiallythe cross-sectional area of said housing and through said gaseous lasingmedium in said firstmentioned chamber ionizing it slightly andtriggering and sustaining a large uniform electrical discharge betweensaid cathode and said anode exciting the lasing medium and creating alarge population inversion allowing stimulation thereby producing a highenergy output.

2. The laser system defined in claim 1, wherein said gaseous lasingmedium is selected from the group consisting of CO N He, selected raregases, and mixtures thereof.

3. The laser system defined in claim 1, wherein said pressurized gaseouslasing medium is at a pressure in the range of about 1 to 10atmospheres.

4. The laser system defined in claim 1, wherein said gaseous lasingmedium is retained in said longitudinally extending chamber of saidhousing at a pressure of about 1 to 10 atmospheres, and selected fromthe group consisting of CO N He, rare gases, and mixtures thereof.

5. The laser system defined in claim 1, wherein said window meanscomprises a layer of material having a thickness in the range of about Ato about 10 mils and selected from the group consisting of aluminum,titanium, and KAPTON.

1. An electron beam-initiated-and-sustained electricaldischarge gaslaser system comprising: a vessel containing a gaseous lasing mediumunder pressure, said vessel comprising a housing defining alongitudinally extending closed chamber therein, Brewster angle windowstransparent to light at frequencies which the medium exhibits gainpositioned in opposite endwalls of said housing, a pair of electrodemeans positioned in spaced relation longitudinally within said chamber,said electrode means extending along a major portion of the longitudinallength of said housing, said pair of electrode means comprising alongitudinally extending cathode and a longitudinally extending anode,said cathode being connected electrically to ground and said anode beingconnected electrically positive with respect to ground, meanselectrically connected to said cathode and anode for impressingthereacross a voltage less than the breakdown voltage of saidpressurized gaseous lasing high energy medium, means for supplying saidchamber with said gaseous lasing medium, window means transparent tohigh energy electrons positioned longitudinally in said housing and inclose proximity to said cathode, said window means comprising a thinlongitudinally extending member defining a portion of a wall surface ofsaid chamber and extending along a major longitudinal portion of saidhousing, said window means being supported by a grill-like member, saidwindow means and said grill-like member being supported by said housingand electrically connected to ground, electron generating meanspositioned longitudinally parallel to and along a major portion of saidhousing adjacent said window means, said electron generating meansincluding a longitudinally extending casing secured to said housing soas to define a chamber in open communication along the longitudinallength thereof with said window means, means connected to said casingfor producing a vacuum in said last-mentioned chamber, said window meansdefining an anode of said electron generating means, cathode meanspositioned longitudinally in said casing in spaced relationship withsaid window means and connected electrically negative with respect toground, means for producing a voltage drop between said cathode meansand said window means including a negative potential power supplyelectrically connected to said cathode means, and means for controllingsaid voltage drop comprising a control grid positioned in said chamberintermediate said cathode menas and said window means and means forcontrolling said control grid, whereby activation of said electrongenerating means directs at least one beam of highenergy electrons alongsubstantially the longitudinal length and throughout substantially thecross-sectional area of said housIng and through said gaseous lasingmedium in said first-mentioned chamber ionizing it slightly andtriggering and sustaining a large uniform electrical discharge betweensaid cathode and said anode exciting the lasing medium and creating alarge population inversion allowing stimulation thereby producing a highenergy output.
 2. The laser system defined in claim 1, wherein saidgaseous lasing medium is selected from the group consisting of CO2, N2,He, selected rare gases, and mixtures thereof.
 3. The laser systemdefined in claim 1, wherein said pressurized gaseous lasing medium is ata pressure in the range of about 1 to 10 atmospheres.
 4. The lasersystem defined in claim 1, wherein said gaseous lasing medium isretained in said longitudinally extending chamber of said housing at apressure of about 1 to 10 atmospheres, and selected from the groupconsisting of CO2, N2, He, rare gases, and mixtures thereof.
 5. Thelaser system defined in claim 1, wherein said window means comprises alayer of material having a thickness in the range of about 1/2 to about10 mils and selected from the group consisting of aluminum, titanium,and KAPTON.